Materials Science Forum Vols. 715-716

Abstract: The five-parameter grain boundary character distribution (GBCD) of a material contains both the grain boundary plane orientation and the lattice misorientation information. This work focuses on generating three-dimensional microstructures that match the full five-parameter GBCD obtained from experimentally observed microstructures. In face-centered cubic metals, the density of high symmetry boundaries is often maximized in order to improve grain boundary dependent properties, such as the resistance to intergranular corrosion and fatigue cracking. Twinning events have been found to be very effective in introducing these high symmetry boundaries that are denoted by low sigma values for Coincident Site Lattice relationships. Therefore, in this investigation, microstructures that contain annealing twins are of particular interest. The statistics that are used to quantify the differences between the synthetic and experimentally observed structures are texture or orientation distribution (OD), GBCD, number and area fractions of S3 and coherent S3 boundaries, S3 cluster distribution, and twin density.

Abstract: Duplex stainless steels (DSSs) are based on the Fe-Cr-Ni system and formed by ferrite (30-70%) and austenite. They have high tensile strength, good toughness and weldability and excellent corrosion resistance including stress-corrosion cracking and resistance to localized corrosion. The increase of the raw materials of the last years, there has been a motivation to develop new stainless steels with lower contents of nickel and molybdenum. Lean duplex stainless steels (LDSSs) are almost Mo free and nickel content lower than 4%. The lean duplex grades are expected to substitute not only 304/316 grades but also other duplex stainless steel grades. LDSSs are used for structural applications and for the less corrosion conditions such as liquor tanks and suction rolls. The aim of the present work was to study the kinetics of recovery and recrystallization of the lean duplex stainless steel 1.4362 during annealing treatment. The material was subjected to hot rolling and cold rolling of 70%, annealing treatment for different times at temperatures from 1000 to 1100°C and subsequently water quenched. Optical microscopy and electron back scattering diffraction (EBSD) were employed to study the evolution of the microstructure during the annealing treatment. After cold rolling austenite exhibited more strain hardening than ferrite. Consequently, the driving force of the austenite for recrystallization is higher. During annealing, recovery took place in ferrite, while the austenite remained nearly unrecovered until beginning of recrystallization. The layered grain morphology produced during cold rolling remained after the annealing treatment. The volume fraction of the phases did not show significant changes with the annealing time. Nevertheless, the volume fraction of austenite decline with an increasing of annealing temperature. After 60 seconds at 1100°C, annealing primary recrystallization had progressed in both phases, which show a bamboo-structure where the grain boundaries ran perpendicular to the phase boundaries. Grain growth progressed for longer annealing times. After 600 seconds, the bamboo-structure started to change for a more globular grain structure, pearl-structure. It continued and became more pronounced at longer annealing times. At lower temperatures, the recrystallization behavior is similar; however, the structure was refined. Recovery is favored by the high stacking fault energy of ferrite and the layered grain morphology. The lean duplex stainless steel 1.4362 shows a similar recrystallization kinetics compare with standard duplex stainless steels.

Abstract: A local curvature multi-vertex model was developed. This model is the straightforward two-dimensional topological network model based on the physical principles which are the curvatures of grain boundaries and the grain boundary tensions at triple junctions. The model was applied to the artificial random microstructure under some conditions of grain boundary characters. The misorientation distribution was changed very little under constant grain boundary energy and mobility, but it was change much under grain boundary character dependent on misorientation. Therefore, in order to discuss actual textures, it is important to take grain boundary characters into account.

Abstract: Stored energy in deformed metals plays an important role during the annealing process by providing the initial driving force for recovery and recrystallization. Many direct or indirect measurement and calculation methods have been used to evaluate the amount and distribution of the stored energy in the past decades. The advent of relatively new analytical techniques such as Electron Back-Scattered Diffraction (EBSD) has permitted the development of mathematical models such as Sub-grain Method, Image Quality (IQ) Method and Taylor Factor Method etc., these new techniques have permitted a much better understanding of the annealing behavior of cold rolled steels. The sub-grain method based on the level of sub-grain structure is used in our study to quantify the stored energy distribution prior to and its evolution during the batch annealing process of cold rolled HSLA steels. Orientation dependent stored energy distribution maps at different annealing stages have been constructed and analyzed. The results of this study show that the stored energy increases with cold rolling reduction ratio and its distribution through the thickness of the steel sample is not uniform due to the inherit inhomogeneous deformation process. The stored energy was continuously consumed during annealing. The amount of γ-fiber was relatively lower than the α-fiber in the specific steel sample, which can have a strong effect on the available driving force for recovery and recrystallization. Hence other structural factors such as precipitation and/or solute drag might become more important in controlling the kinetic behavior of the steel during annealing.

Abstract: The conditions for the nucleation of abnormal grain growth were investigated using a three dimensional vertex dynamics model. Potentially abnormal growing grains characterized by their size and topological class, respectively and embedded in an isotropic grain ensemble were subjected to annealing varying their grain boundary properties. The simulation results indicate that the classical mean field approaches underestimate the role of the grain boundary energy advantage, while the impact of a mobility advantage is overestimated.

Abstract: The recrystallization behavior of 50% cold rolled Fe-22%Mn-0.376%C alloy during annealing at 560°C, 630°C and 700°C was investigated. Microhardness tests were applied for characterization of the recrystallization kinetics, X-ray diffraction and EBSD measurements were utilized to characterize the crystallographic texture and the grain microstructure. The obtained experimental data were evaluated in terms of the JMAK model. The obtained values of the Avrami exponent varied in the range between 0.70 and 1.37. The inhomogeneous grain microstructure after recrystallization is interpreted in terms of non-randomly distributed nuclei. Shear bands, lamellar lines intersecting with mechanical twins and grain boundaries with localized high misorientation gradients were identified to be preferential nucleation sites. No pronounced texture was observed after annealing at 630°C.

Abstract: t has been shown by computer simulations that the MacPherson-Srolovitz relation predicts accurately the growth rate of a grain undergoing ideal grain growth. However, since a finite mobility of the boundary junctions (triple lines and quadruple junctions) affects the evolution of a granular system, it is necessary to modify this equation in order to take into account their effect. In the present contribution, an equation which allows considering these factors is presented and used to modify the von Neumann-Mullins and MacPherson-Srolovitz equations. In order to corroborate these equations two and three dimensional network model simulations were performed. The results showed a very good agreement with the theoretical approaches for both dimensions and all topological classes except those near the classes of zero growth rate in 3D. The reason is that the proposed function is very sensitive to small changes of the finite mobility of the junctions.

Abstract: ncreasing demand for automotive vehicles with reduced weight, improved crashworthiness and passengers safety has steamed the research of new Twinning Induced Plasticity (TWIP) steels. In this work the effect of annealing between 400 and 900°C on the microstructure and mechanical properties of hot and cold rolled 0.06C-24Mn-3Al-2Si-1Ni (wt%) steel with TWIP effect was investigated. The results have shown that steel exhibits fast recrystallization kinetics with a low amount of recovery, which results in a high driving force for the former. Mechanical properties were determined using Vickers microhardness and tensile tests. Tensile strength of 670 MPa with 54% of total elongation, and strain hardening exponent of 0.57 were reached after annealing at 900°C.

Abstract: Dynamic recrystallization (DRX) behavior in a coarse columnar-grained Cu-0.65Sn-0.025P and Cu-0.025P (mass%) alloys were systematically investigated by compression tests at temperatures between 1073 K and 1253 K and at true strain rates from 2 x 10^-4 s^-1 to 2 x 10^-1 s^-1 in vacuum. As a model sample, an orientation-controlled bicrystal having [0 1 twist 18^o boundary was prepared and also hot deformed. Appearance of the peaks stress, where DRX onsets, was much delayed in Cu-Sn-P alloy compared with that in Cu-0.025P alloy. The onset of DRX was, therefore, obviously impeded by the small addition of Sn to Cu-P. While nucleation of new grains took place almost at random in Cu-Sn-P when strain rate was high enough, it tended to appear more preferentially at grain boundary with decreasing strain rate and with increasing temperature. The most of new grains were annealing twins formed behind the migrating grain boundary. Because grain boundary migration took place more extensively with increasing temperature and with decreasing strain rate, the preferential nucleation at grain boundary became more significant.